In a known method for sewage sludge treatment, the digested sludge is subjected to treatment in an apparatus which includes a combustion chamber, a dryer, a cyclonic dust precipitator and a suction blower, all connected in series. It is necessary to label the inlet temperatures of material admitted to the dryer below approximately 550.degree. C. This requires that a substantial amount of cold air is added to the hot gases generated in the combustion chamber prior to admission to the dryer. As a result, these gases acquire an excess of air leading to an air factor .lambda. between 2 and 3, i.e., a relatively high value. Even if the outlet temperature of the dried vapors is held at the relatively low temperature of 100.degree. C., the thermal efficiency of the drying process is relatively low. Furthermore, the high air factor .lambda. means that the dryer is operated with gases which have an oxygen content of more than 13%. Such gases cause numerous dryer fires and even explosion-like combustion within the dryer. Furthermore, the known dryer apparatus connected in the manner described above causes emissions of dust and foul-smelling substances into the atmosphere which is contrary to the goals of environmental protection. The dust contained in the gas could be eliminated by placing an air scrubber between the cyclonic precipitator and the suction blower but the unpleasant odors are generally not removed from the gas by scrubbing.
Another known method for sludge drying and treatment has a provision for returning sludge vapors due to drying to the inlet of the dryer to be admixed there with the hot gases coming from the combustion chamber. This method results in improved heat economies and a relatively lower air excess factor without increasing the temperature of the dryer beyond permissible limits. However, the aforementioned emissions of dust and odor-causing vapors are not eliminated. Accordingly, the known sludge drying apparatus and method can be used only for drying pure mineral sludges in which the drying vapors are odor-free.